Not applicable.
1. Field of the Invention
The present invention relates to apparatus and methods for imaging formation zones surrounding a borehole.
2. Background Art
Energy exploration and exploitation using boreholes drilled into earth formations require the monitoring and evaluation of physical parameters, such as resistivity and conductivity of Earth formations surrounding a borehole.
Methods of electromagnetic field excitation may be generally classified as frequency domain excitation and time domain excitation. In frequency domain excitation a continuous wave signal is transmitted, normally at a fixed frequency, although the transmission could be a plurality of superimposed frequencies. For time domain excitation, the signal, which may be a square wave, or a pulsed, triangular or a pseudo random binary sequence signal, is abruptly switched.
A limitation of frequency domain (continuous wave) excitation is the strong coupling between the transmitter and the receiver. This coupling, known as the direct mode, arises because of the detection by the receiver of the magnetic field transmitted directly from the transmitter to the receiver. The direct mode signal may be stronger than the signal received from the formation, and make it difficult to accurately measure the signal received from the formation. Methods of enhancing the resolution of the frequency domain method include the use of multi-coiled devices, such as conventional borehole induction tools, focused permanently on certain spatial areas of the formation. Such methods also include the use of tools such as an array-type induction or laterolog measurement tool to generate an array of measurements, and the application of multi-target processing techniques to the array of measurements to provide numerical focusing on selected regions of the formation. However, the net signal resulting from these multi-target processing techniques is small compared to total measured signal.
When utilizing time domain excitation, the excitation current is abruptly switched off, thereby producing a transient signal which is detected by the receiver. And because the transmitter signal is no longer being generated during the time when the transient signal is being detected, the received signal may be filtered to remove any remaining influence of the direct mode signal. The direct mode signal, which contains no information about the formation resistivity/conductivity, is excluded from the transient measurement.
The ability to separate in time, in the detected signal, the response of different spatial areas of the formation, is a significant attribute of the transient method. In accordance with Lenz""s rule, upon switching off the transmitter current, induced currents in response to the change in the transmitter current. The geometric distribution of the induced currents is similar to the transmitter current which was switched off. After the transmitter current is switched off, the current begins diffusion to the outside formation. This diffusion is followed by attenuation and dispersion in which the spatial resolution in the later time stage becomes significantly reduced. However, transient field data in the later time stages have proved to be more sensitive to the distant formation resistivity than frequency domain or DC data.
Transient electromagnetic measurement techniques have been utilized in mining operations for making resistivity/conductivity measurements in which a large surface dipole antenna (often several hundred meters in length) is utilized with electromagnetic receivers located in a borehole to make measurements in zones in the Earth surrounding the borehole and between the borehole and the Earth""s surface. Such use for mining operations is fairly common. More recently, geophysical operations have utilized such large surface dipole antennas on the Earth""s surface, as shown in U.S. Pat. No. 5,467,018, which issued to Ruter et al. on Nov. 14, 1995. U.S. Pat. No. 5,467,018 is incorporated herein by reference for all purposes.
Until recently, modeling of the transient response had been restricted to a fairly simple, approximate model. However, methods are now known for developing a realistic model for borehole transient electromagnetic response. See, for example, Tabarovsky, L. A., Goldman, M. M., Rabinovich, M. B., Strack, K.-M., 1996, 2.5-D Modeling in Electromagnetic Methods of Geophysics, Journal of Applied Geophysics 35, 261-284. Parallel to such developments in the area of numerical modeling, the electronic capabilities in high power switching, amplifier design and data transmission have improved, thereby making a time domain borehole system feasible.
The limitation on the radial depth from which measurements may be made with the transient electromagnetic method is determined primarily by the signal-to-noise of the measurements, which is related to the impulse energy that can be generated. Further, the interpretation of the measurements is simplified if the structure of the formation boundaries has been obtained, or at least approximated, from other geophysical data, such as gravity, seismic, borehole log or geologic survey data. This information can be used to keep certain parts of the Earth parameters fixed while other parameters are interpreted from the data.
DC excitation may also be used, but the measured signal is a composite signal comprising a mixture of configurations from different regions of the subsurface. The resolution is accordingly reduced.
U.S. Pat. No. 5,955,884, which issued on Sep. 21, 1999 to Payton et al. discloses a system in which a logging tool includes at least one electromagnetic transmitter and at least one electric transmitter for applying electromagnetic energy to the formation at selected frequencies and waveforms. The electromagnetic transmitter is preferably a three axis transmitter comprising three orthogonal coils for generating the magnetic field, and the electric transmitter is preferably a three axis transmitter comprising three orthogonal electric dipole antennae for generating the electric field. U.S. Pat. No. 5,955,884 is incorporated herein by reference for all purposes.
Other issued patents which may relate to the subject matter of this invention include without limitation U.S. Pat. Nos. 5,543,715; 5,841,280; 5,862,513; 5,883,515; 5,870,690; 6,147,496, which patents are incorporated herein by reference for all purposes.
The invention is a method for generating an image of earth formations penetrated by a wellbore. The method includes generating an initial model of the earth formations using formation resistivity measured by a direct current signal. A response to the initial model of an instrument used to make the direct current resistivity measurements is calculated. The calculated response is compared to the measurements of resistivity. The model is adjusted, and the calculating and comparing are repeated until a difference between the calculated response and measurements reaches a minimum. The adjusted model is refined based on resistivity measurements made using an electromagnetic measuring instrument, and the refined model is constrained using acoustic velocity measurements.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.